1. Field of the Invention
The present invention relates to a process for preparing carbon-black-filled rubber powders using rubbers prepared in and/or present in organic solvents, and to the resultant powders and their use in the rubber-processing industry.
2. Description of the Background
U. Gxc3x6rl and K. H. Nordsiek in Kautsch. Gummi Kunstst. 51 (1998) 250; and U. Gxc3x6rl and H. Lauer in Gummi, Fasern Kunstst. 53 (2000) 261 and R. Uphus, O. Skibba, R. H. Schuster and U. Gxc3x6rl in Kautsch. Gummi Kunstst. 53 (2000) 276 disclose the reasons for, and benefits from, the use of rubber powders, and also disclose possible processes for preparation of the rubber powders. The interest in pulverulent rubbers can be explained as an inevitable consequence of the processing technology used in the rubber industry, where rubber mixtures are prepared at high costs because of the time, energy and manpower needed to produce the rubber powders. The main reason for this is that crude rubber material is procured in the form of bales and the remaining constituents of the rubber material which is subject to vulcanization have to be incorporated into the rubber phase.
Comminution of the bale and intimate mixing with fillers, mineral oil plasticizers, and vulcanization auxiliaries takes place on rolls or in internal mixers, in a number of process stages. Between the stages, the mixture is generally cooled on a batch-off system, laid out in milled sheet form on pallets, and put into intermediate storage. Downstream of the internal mixers or rolls the rubber material is subjected to appropriate extrusion or calendering processes.
The only way to avoid this very complicated method of rubber processing is to use completely new process technology. For some time, therefore, there have been discussions on the approach of using free-flowing rubber powders which allow rubber mixtures to be processed as simply and rapidly as thermoplastic powders or pellets.
Rubber products are generally produced by the rubber industry, by polymerization of various and appropriate monomers, by two fundamentally different processes:
a) Polymerization in Water (Emulsion Polymerization)
As suggested by the title of this process, this process embodiment involves monomer polymerization in water initiated by a free-radical, with the aid of suitable initiator molecules, to give polymers of high molecular weight. If emulsifiers are present, the form in which the rubber molecules are present after the polymerization is as finely divided droplets in water. The term latex or rubber emulsion is used in this context. The latex of emulsion produces the raw rubber material, in turn, for the preparation of rubber powder from the aqueous phase, the powder generally being produced by acid-catalyzed coprecipitation after addition of fillers.
U. Gxc3x6rl and K. H. Nordsiek in Kautsch. Gummi Kunstst. 51 (1998) 250; U. Gxc3x6rl and H. Lauer in Gummi, Fasern Kunstst. 53 (2000) 261; R. Uphus, O. Skibba, R. H. Schuster and U. Gxc3x6rl in Kautsch. Gummi Kunstst. 53 (2000) 276; DE 28 22 148, DE 37 23 213, DE 37 23 214, EP 99 911 7844.5, DE 198 154 53.4, DE 198 43 301.8 and DE 100 56 636.0 are all relevant literature showing developments in this area.
It should merely be stated that the best known rubbers which are prepared in, or are present in, water include natural rubber (NR), emulsion styrene-butadiene rubber (ESBR), nitrile rubber (NBR), and chloroprene rubber (CR). All of these types of rubber may be modified with fillers, e.g. industrial carbon blacks or precipitated silicatic fillers, to give rubber powders.
b) Polymerization in an Organic Solvent
The second large group of rubbers is that of products which are generally polymerized anionically in an organic solvent and therefore are also present in this solvent after the polymerization. Important rubbers prepared in this way include styrene-butadiene rubber (LSBR), butadiene rubber (BR), butyl and halobutyl rubbers, and also ethylene-propylene rubbers with (EPDM) or without (EPM) a copolymerized tercomponent.
The process for preparing a rubber powder from solution polymers, which are dissolved in organic solvent, has to take account of this completely different type of rubber starting material. In contrast to the aqueous rubber emulsion, the following new problems arise:
Direct addition of the filler, in particular of the carbon black, to the rubber solution is problematic, because the filler (lipophilic) absorbs all of the solvent and the rubber/filler mixture therefore clumps, making it difficult to prepare a free-flowing rubber powder. The high adsorptive forces generated by the carbon black, furthermore, bind the solvent, and it is therefore difficult to remove the solvent quantitatively from the product, even under conditions of distillation. A product of this type used in the rubber industry would pose a risk of continuing unacceptable emission of traces of solvent during processes.
Direct preparation of the rubber powder from the polymer solution makes it difficult to take measures to control (reduce) tackiness, this being a precondition for a free-flowing product which is capable of being conveyed and silo storage.
In the case of rubber powders made from aqueous rubber emulsions, this could be achieved by applying a separate filler layer around each grain of rubber during coagulation of the latex with certain adjustments to pH.
In an organic solvent, on the other hand, acid-catalyzed coagulation is not possible, and nor therefore is the application of an effective release layer around each grain of rubber. The only method which remains in this case is subsequent powdering of the product, with the risk that this release agent will separate from the grain during conveying and silo storage, resulting in increased tack and then serious problems during processing. Another consequence could be variations in filler level. Another risk when using release agents which are not a constituent of a rubber mixture is that of product contamination, possibly leading to unacceptable impairment of performance profile.
This means that preparation of a carbon-black-filled rubber powder based on solution polymers which are dissolved in an organic solvent with the desired product properties of flowablity, free-flowability, capable of being conveyed and silo storage, storage stablity, etc. can be achieved only if it is possible to combine the abovementioned aspects in a preparation process, and this in principle appears to be possible only by way of a two-phase system (org./aqueous).
The patent literature describes in detail the preparation of carbon-black-filled rubber powders from organic rubber solutions, using a number of processes. DE 28 22 148 describes the preparation of rubber powders both from aqueous rubber emulsions and from rubber solutions. In the latter case, a carbon black suspension treated with acid, Al2(SO4)3, and water glass is heated to the boiling point of the solvent, the pH is readjusted, and then the rubber solution is added. While the solvent evaporates, the rubber coagulates on the carbon black under the action of the additives. The rubber powder may be obtained after solid/liquid separation and drying, but no other measures have been taken to inhibit tack and thus ensure that, even after long storage times, the material is capable of being conveyed and being stored in a silo. In practice it has been found that unless precautionary measures are taken a product prepared in this way has at best short-term capability to remain free-flowing, the result therefore being problems with silo storage and automatic feeding of mixers.
DE 21 35 266, DE 22 14 121, DE 26 54 358 and DE 24 39 237 describe processes which first convert the organic rubber solution into an aqueous emulsion with the aid of large amounts of emulsifiers. Carbon black filler suspended in the water is added to this emulsion, and the entire emulsion is made to flow into a heated sodium water glass solution. The result is coprecipitation of rubber and filler with simultaneous removal of the solvent. The process is therefore again based on the principle of coagulation by alteration of pH. The use of the emulsifiers overcomes the problem of the incompatibility of the organic polymer solution phase with the aqueous filler suspension phase. However, the use of such large amounts of emulsifiers is extremely problematic. Firstly, some of the material remains in the finished rubber powder, in some circumstances resulting in a disadvantageous effect on vulcanization-related properties (foreign constituents in the mixture), and secondly the remainder passes into the waste water, which is therefore subject to severe pollution by organic constituents. With this method of preparation, again no adequate measure is described for reducing the tack of the rubber powder. Any such measure is also unlikely to be capable of implementation by this preparative route.
DE 22 60 340 describes a markedly different process, in which the rubber solution with the filler, which by this time is present in dispersion in the organic solvent, is added in a single shot to the rubber solution. The resulting liquid material is then subjected to extremely intimate mixing. There is therefore no preparation of a separate carbon black suspension in water. The solvent is then flash-evaporated by reducing the pressure, using temperatures of up to 280xc2x0 C. Remaining solvent constituents are removed in a conventional drier. The advantage of this process is certainly that practically no additives have to be used, and the proportion of foreign constituents in the product can therefore be kept low. In addition, there is no production of waste water requiring expensive purification because of organic contaminants.
However, a problematic feature of this process is the use of high pressures and temperatures, which on an industrial scale require very complicated means for their implementation. There is also the risk that sensitive rubbers will be damaged by the rigorous conditions (polymer chain degradation), and that therefore vulcanization-related properties will suffer. Another problem is that solvent residues remain in the product and also have to be removed in a drying step. Indeed, it is doubtful (see page 3) to what extent this is quantitatively possible, since the carbon black is highly adsorptive. The process also fails to address the question as to how the tack of a product obtained in this way can be reduced, and a lastingly free-flowing product capable of being conveyed and silo storage reliably obtained.
Accordingly, one object of the present invention is to provide a process for preparing a free-flowing rubber powder composed of a rubber/carbon black composite based on solution polymers, where the powder gives a product which meets processing requirements of being free-flowing, capable of being conveyed and silo storable and being capable of automatic feeding and which also meets rubber-technology requirements.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a process for preparing fine-particle, free-flowing rubber powders composed of carbon black fillers and a rubber, which comprises:
having prepared the rubber by solution polymerization which is present in an organic solvent solution:
a) first suspending a carbon black in an appropriate amount as a filler for the rubber in water by means of a high-shear mixing device (e.g. Ultra-Turrax), without any other additives, and setting the solids content of the suspension to 0.5-15%,
b) separating some of the suspended carbon black from the amount of carbon black of step a), the amount of carbon black separated (in suspension) ranging from 0.5-20%, preferably from 1-10%, based on the total amount of carbon black in the finished product,
c) heating the remaining suspension which comprises most of the carbon black to a temperature approximately equal to the boiling point of the organic solvent in which the rubber has been dissolved,
d) introducing the rubber solution into the carbon black suspension, with stirring, and at the same time removing solvent by distillation at atmospheric pressure or in vacuo, while the amount of thermal energy supplied being sufficient to keep the temperature of the reaction mixture in the region where evaporation of the solvent is possible, and in this process converting the rubber/filler composite from a two-phase system to a single-phase system,
e) removing the organic phase by distillation in water, and not by pH-initiated acid- or base-catalyzed coagulation in water, to transfer the rubber into the aqueous carbon black suspension thereby forming the rubber/filler composite, whereupon, once the solvent has been removed, the product is a rubber/carbon black composite in an aqueous single-phase system;
f) obtaining the product in the form of a rubber/carbon black composite in an aqueous single-phase system after removal of the solvent,
g) mixing the carbon black suspension separated in step b) into the aqueous single-phase system, with stirring,
h) adding from 1-15 parts, preferably 5-10 parts of a rubber emulsion (e.g. ESBR or NR), based on a total of 100 parts of rubber, to the suspension g), thereby forming a suspension having a pH of 2-7, preferably 2.5-5, by the addition of a Brxc3x6nsted or Lewis acid, e.g. sulfuric acid, aluminum sulfate etc., to the suspension,
i) the result of the processing of steps g) and h) being that, around each grain of rubber powder, an effective carbon black release layer which is mechanically anchored to the solution-polymer-based carbon black/rubber core particle and which subsequently is effective in suppressing the tack of the material in the dried finished product, is formed,
j) removing most of the water from the material being processed in steps a) to h) by a suitable solic/liquid method of separation, and
k) drying the moist product to a residual moisture levelxe2x89xa62%.